The invention relates to an article of manufacture, in particular a component of a gas turbine, having a base body and a thermal barrier coating formed thereon. The invention also relates to a process for applying a thermal barrier coating to an article which can be exposed to a hot aggressive gas and has a base body, in particular a metallic base body.
U.S. Pat. No. 5,238,752 describes a thermal barrier coating system having an intermetallic bond coating. The thermal barrier coating system is applied to a metallic base body, in particular to a Crxe2x80x94Co steel for an aircraft engine blade. An intermetallic bond coat, in particular of a nickel aluminide or a platinum aluminide, is applied directly on top of that metallic base body. The bond coat is adjoined by a thin ceramic layer of aluminum oxide, to which the actual thermal barrier coating, in particular made from yttrium-stabilized zirconium oxide, is applied. This ceramic thermal barrier coating of zirconium oxide has a columnar structure, the rod-shaped columns being oriented substantially perpendicular to the surface of the base body. This is intended to improve the ability to withstand cyclic thermal loading. The thermal barrier coating is deposited on the base body by means of an electron beam PVD (physical vapor deposition) process. The zirconium oxide is thereby vaporized out of a metal oxide body using an electron beam gun. The process is carried out in a corresponding device, in which the base body is preheated to a temperature of approximately 950xc2x0 C. to 1000xc2x0 C. During the coating operation, the base body is rotated at constant speed in the jet of metal oxide.
An electron beam PVD process for producing a ceramic coating is also described in U.S. Pat. No. 5,087,477. There, the ceramic coating has a layer thickness of between 250 and 375 xcexcm.
U.S. Pat. Nos. 4,405,659 and 5,514,482 each describe components, in particular gas turbine blades, made from a nickel-base or cobalt-base alloy, and in each case a ceramic thermal barrier coating of columnar structure is applied to these components. The mean diameter of the columns is in that case over 2.5 xcexcm, the layer thickness amounting to approximately 125 xcexcm. The ceramic thermal barrier coating is applied by means of an EB (electron beam) PVD process.
International PCT publication WO 98/13531 describes a component, in particular a gas turbine blade, which has a ceramic thermal barrier coating of columnar fine structure on a metallic base body, the mean column diameter being less than 2.5 xcexcm. This small mean column diameter for layer thicknesses of the order of magnitude of over 100 xcexcm which are used in gas turbine construction is achieved by means of a reactive gas flow sputtering process. In this process, an ionizable gas is passed through a hollow cathode and, on account of the voltages prevailing in the hollow cathode, is ionized, and is thus accelerated toward the inner wall of the hollow cathode. The hollow cathode has the coating material, in particular metallic zirconium, on its inner wall, this material being thrown out by the ions and transported toward the base body which is to be coated.
U.S. Pat. No. 5,350,599 describes a thermal barrier coating for a turbine blade which has a plurality of layers positioned on top of one another. The outer surface layer is of erosion-resistant design, while the layer beneath it is of porous design. Both layers are made from ceramic material and are applied successively by means of a PVD process. The porous or sealed structure of the erosion-resistant outer layer is obtained by varying the process parameters during the coating process. The application of the outer erosion-resistant layer is intended to protect the turbine blade from damage caused by erosion.
European published patent application EP 0 139 396 A1 describes a coating system for a turbine blade in which different coatings are applied in different surface regions, specifically as a function of the temperature occurring on the turbine blade. In this case, a distinction is drawn between a hot end and a cold end of the turbine blade. The various coatings are adapted to the different temperature requirements in particular in terms of their ductility and their creep behavior. For this purpose, they have different chemical compositions. This requires a transition layer to be arranged between adjacent layers of different compositions.
The object of the present invention is to provide an article of manufacture that can be exposed to hot aggressive gases which overcomes the above-noted deficiencies and disadvantages of the prior art devices and methods of this general kind, and which is formed with a thermal barrier coating that satisfies the attendant requirements. A further object of the invention is to outline a process for coating an article with a thermal barrier coating.
With the above and other objects in view there is provided, in accordance with the invention, an article of manufacture that is to be exposed to hot aggressive gas. The article comprises a base body or substrate having a first surface region and a second surface region, and a thermal barrier coating of uniform chemical composition applied on the base body, the coating having a fine structure in the first surface region different from a fine structure in the second surface region.
In other words, the article of manufacture, in particular a component of a gas turbine, which can be exposed to a hot aggressive gas has a base body and a thermal barrier coating of uniform chemical composition applied to the base body. The coating has different microstructures in a first surface region and in a second surface region and is thus adapted to meet the requirements imposed by the thermomechanical stresses on the product which in each case prevail locally or are to be expected for the particular intended use. The two surface regions lie in the same surface plane or surface layer. They are in particular arranged next to one another. In the surface regions, the thermal barrier coating has a thermal barrier material which is uniform in terms of its chemical composition. The thermal barrier coating can therefore be referred to as a single-material barrier coating. This has the considerable advantage over multimaterial barrier coatings that problems which are inherent to a material transition are avoided. At the same time, on account of the different microstructures, the respective surface regions are designed for the expected loads.
In this case, it is likewise additionally or alternatively possible, in regions which are of geometrically different design, in particular with regard to surface curvature, convexity or concavity, if the local thermomechanical load to be expected is the same, for the fine structure in the geometrically different surface regions still to be of substantially identical form. This particular form of the coating is advantageous in particular in the case of curved components which are exposed to a hot gas stream which leads to locally different thermomechanical loads, since the thermal barrier coating is locally adapted to the thermomechanical stresses, such as temperature and the action of forces as a result of impinging particles, which occur. As a result, it is possible to locally influence, in particular extend, the service life of the thermal barrier coating in a controlled way, so that the duration of use and the service life of the product are also extended.
In accordance with an added feature of the invention, the thermal barrier coating has a fine structure with ceramic columns which are oriented substantially perpendicular to the surface of the base body. In this case, the ceramic columns may have a diameter of a few microns (xcexcm) at a layer thickness of up to 100 xcexcm or more. A fine structure with ceramic columns is particularly advantageous, since it is able to follow thermal expansion of the base body particularly in the event of cyclic temperature changes without being damaged.
In accordance with an additional feature of the invention, the ceramic columns in the first surface region have a smaller mean diameter than in the second surface region. Consequently, the thermal barrier coating in the first surface region can generally have a finer fine structure than in the second surface region, with the result that in particular this region of finer fine structure can withstand higher temperatures, in particular temperature change stresses, The thermal barrier coating in the first surface region, which is exposed to higher thermomechanical stresses than the second surface region, preferably has a denser fine structure than the thermal barrier coating in the second surface region.
The article is preferably a component of a thermal machine, in particular of a gas turbine, such as a stationary gas turbine which is used in power plant engineering or an aircraft engine turbine. The product may in this case be designed as a heat shield for a combustion chamber or as a turbine blade, a turbine rotor blade or a turbine guide vane. A turbine blade has a leading edge and a trailing edge, with a pressure side and a suction side arranged on opposite sides between these edges. The leading edge is the area of a turbine blade which is subjected to the highest thermal loads and, moreover, is also exposed to intense erosion conditions on account of the hot gas incident upon it. Locally similar thermomechanical stresses prevail on the suction side and the pressure side. Consequently, a turbine blade preferably has a thermal barrier coating of substantially identical fine structure (second surface region) on the suction side and the pressure side. A finer fine structure of the thermal barrier coating (first surface region) is preferably provided on the leading edge, so that this coating meets the demands imposed by the thermomechanical stresses prevailing in that area. A thermal barrier coating of this type is better able to withstand the thermomechanical stresses than the thermal barrier coating on the suction side and pressure side.
The thermal barrier coating is preferably ceramic. It may contain zirconium oxide (ZrO2) or another ceramic material which is suitable for use at high temperatures, in particular a metal oxide. A zirconium oxide is preferably partially or completely stabilized with yttrium oxide (Y2O3) or with another oxide of a rare earth element. Alternatively, the thermal barrier coating may also comprise a compound selected from the group consisting of the refractory perovskites, e.g. LaAlO3, CaZrO3, or the spinels, e.g. MgAl2O4, MgCr2O4 or AB2O4, where A represents an element selected from the group consisting of Ni, Co, Ti; B represents Al or Cr, and O represents oxygen.
In accordance with a preferred embodiment, the base body is of metallic design. For applications at high temperatures with corresponding demands imposed on corrosion resistance, nickel-base and/or cobalt-base alloys, as described, by way of example, in U.S. Pat. No. 4,405,659, inter alia, are particularly suitable.
In accordance with another feature of the invention, there is provided an adhesion promoter layer between the base body and the thermal barrier coating. The adhesion promoter layer may be made from an alloy comprising chromium, aluminum, yttrium and/or one of the elements from group IIIb of the periodic system, including the actinides and the lanthanides, and also, in addition or as an alternative, rhenium, while the predominant remainder of the alloy may comprise iron, cobalt and/or nickel. Yttrium-containing alloys of this type are described in the literature by the term xe2x80x9cMCrAlYxe2x80x9d alloy. Alloys which contain significantly more rhenium than yttrium may be referred to as xe2x80x9cMCrAlRexe2x80x9d alloys. An oxide layer, in particular of aluminum oxide, chromium oxide and/or gallium oxide, may be provided between the adhesion promoter layer and the thermal barrier coating. An oxide layer of this type may already have been applied as an oxide or may be formed as a result of oxidation under thermal conditions (thermally grown oxide TGO).
With the above and other objects in view there is also provided, in accordance with the invention, a method of forming a thermal barrier coating on an article to be exposed to a hot aggressive gas. The method comprises the steps of providing a base body and forming on the base body a thermal barrier coating of uniform chemical composition, and thereby forming the thermal barrier coating in a first surface region of the base body with a first fine structure and forming the thermal barrier coating in a second surface region with a second fine structure, and defining the first and second surface regions in accordance with a locally expected thermomechanical loading when the article of manufacture is in use.
In other words, the object relating to a process for applying a thermal barrier coating to a product which can be exposed to a hot aggressive gas is achieved in that a thermal barrier coating of uniform chemical composition is applied to the base body in such a manner that, in a first surface region of the substrate, the thermal barrier coating is formed with a first fine structure and in a second surface region the thermal barrier coating is formed with a second fine structure, in accordance with the locally expected thermomechanical loading when the product is in use.
Consequently, the process provides a product which takes into account the local thermomechanical stresses occurring on the product. In particular, it is thus possible to make the thermal barrier coating more resistant at a region which is subjected to high loads (first surface region) than in a second surface region which is subjected to lower thermomechanical loads. As a result, it is also possible to produce a respective thermal barrier coating of substantially uniform fine structure in a controlled manner in local surface regions which are exposed to substantially identical thermomechanical conditions. This is particularly advantageous if the geometric design of such second surface regions differs, in particular if the curvature differs significantly in terms of size and/or direction (concave, convex).
The process for producing a thermal barrier coating is preferably carried out as an electron beam physical vapor deposition (EB-PVD) process or as a reactive gas flow sputtering process, as described, for example, in WO 98/13531 A1. In the case of the processes for the physical vapor deposition of a thermal barrier coating, the coating process is determined by various process parameters, such as the temperature of the product to be coated, the power of an electron beam gun, acceleration of the particles to be deposited toward the product, gas flow in a coating chamber, supply of oxygen, partial pressure of a reactive gas, overall pressure in the coating chamber, heating capacity of a heater device, rotational speed of the product, relative movement between product and a particle jet which forms the thermal barrier coating and other process parameters. In the process, individual process parameters or a plurality of process parameters are controlled or regulated according to the local demands imposed on the thermal barrier coatings in such a way that the thermal barrier coating is formed according to local requirements.
The process is preferably applied to a blade of a turbomachine, in particular of a gas turbine, in which case the thermal barrier coating on the suction side may have substantially the same fine structure as on the pressure side. In addition or as an alternative, in the case of a blade it is also possible to apply a more resistant thermal barrier coating, in particular with a finer fine structure, to a leading edge, which is exposed to elevated thermomechanical stresses, in particular to erosion, than at surface regions which are exposed to lower stresses. Surface regions which are subjected to lower stresses may include the suction side, the pressure side or the trailing edge of the blade. In this case, the process parameters, such as for example the movement of the product (the turbine blade), which may be executed as a simple rotation of the product about an axis and as a pivoting and/or inclination-changing movement, the local coating rate on that surface of the product which faces a particle deposition crucible, and further process parameters can be adapted to one another in such a way that the locally desired layer structure which is appropriate for the stresses imposed is produced. Parameters which may have an effect in this context include, inter alia, the coating rate, the capacity of a vaporizer gun in EB-PVD processes, the deflection of the vaporizer gun electron beams, the overall pressure in the coating chamber, the partial pressure of a reactive gas, the temperature of the product, the nature of the movement and the relative velocity of the product during the vapor deposition.
By way of example, in the case of a turbine blade a thermal barrier coating which is appropriate for the particular stresses can be produced by linking the vaporization capacity to the rotation of the turbine blade. In this case, when the leading edge of the blade is facing the vaporization crucible, the rotational speed can be increased in combination with a reduced vaporization capacity, so that the coating rate is locally reduced and, at the same time, a dense coating structure is produced. When, for example, the pressure side of the blade is facing toward the vaporization crucible, the rotational speed can be reduced and the vaporization capacity can be increased, so that the layer structure of the thermal barrier coating is less dense and so that, by simultaneously adapting the process for the suction side accordingly, the thermal barrier coating on the suction side and on the pressure side has a substantially identical fine structure.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a product with a thermal barrier coating and a method of producing a thermal barrier coating, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.